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Introduction
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<H2 CLASS="section"><A NAME="htoc231">17.1</A>&nbsp;&nbsp;Introduction</H2>
This chapter introduces the control facilities that distinguish the
ECL<SUP><I>i</I></SUP>PS<SUP><I>e</I></SUP> language from Prolog by providing a computation
rule that is more flexible than simple left-to-right goal selection.
The core feature is the ability to suspend the execution of a goal
at some point during execution, and resume it under certain conditions
at a later stage.
Together with attributed variables, these facilities are the
prerequisites for the implementation of constraint propagation
and similar data-driven algorithms.<BR>
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